You need to write to the Client Characteristics Configuration Descriptor (CCCD) of that particular characteristics in the GATT server device (PSoC4 BLE) which is being used for sending the Notifications from the APP INVENTOR (GATT Client).

Please refer the CySmart Android App for enabling the notifications ( Writing to the CCCD) from below link.

Also, refer the PSoC 4 BLE Server Project ( Peripheral ) from below link to handle the CCCD write request. ( Here a Central project is also present , you can understand the functionality by going through both the projects )